1. Field of the Invention
The present invention relates generally to novel arrangements, including both systems and methods, for generating narrow beams of traveling wave fields in space, and more particularly pertains to several embodiments for integrated radiation cavities (either LASER or MASER cavities) designed to generate in their own medium a Bessel mode diffraction free beam.
Much of the disclosure herein is applicable to all types of waves as described by the basic Helmholtz wave equation, including electromagnetic waves such as radio frequency, microwave, infra-red, optical and x-ray waves, relativistic and nonrelativistic quantum waves associated with particle waves, such as electron, neutron, proton, atom and other quantum particle waves, and further including physical elastic waves such as material deformation waves and longitudinal waves including acoustical waves.
2. Discussion of the Prior Art
Current state of the art techniques to concentrate a wave or form a parallel beam are generally successful only over a very limited range of beam propagation. This range is conventionally related inversely to the degree of concentration. This inverse relationship arises primarily because all wave fields are subject to diffraction (i.e., beam spreading).
The arrangements of the subject invention have several advantages over all prior art techniques currently in use, with a principle advantage thereof being greatly improved resistance to diffraction.
Two methods exist in the current state of the art for generating narrow beams, focusing and collimation. Due to the ever present effects of diffraction, a focus is never perfect. Instead, a focus is characterized as a finite region over which a beam has a minimum radius. The distance along the lens axis, on one side or the other of the focus, where the beam exhibits significant convergence is called the depth of field of the focus. The depth of field of a focus is generally limited by the sharpness of the focus. That is, a very small focal spot can be achieved only at the expense of depth of field.
All light waves, such as those radiated by the sun, lamps and lasers, can be collimated as well as focused. Collimated (parallel) beams are generally preferred because they have much greater depth of field than focused beams, although they are less bright. Collimation is normally accomplished by a series of aligned apertures, which are basically just holes in opaque screens, which allow the light through along just one direction. A sequence of aligned holes along a collimation axis of a beam provides the normal manner of creating a well-defined parallel or collimated beam.
Unfortunately, diffraction affects collimation adversely just as it does focusing. The effects of diffraction on collimation can be described with the explanation that a wave field bends outwardly from the edges of a hole as it proceeds therethrough, and thus the resulting beam is not as well collimated. FIG. 1 illustrates the characteristic behavior of waves traveling through holes. The diffractive bending of water waves that are entering a narrow harbor or passing by a jetty can be shown easily in aerial photographs thereof because of the large scales involved, but the bending of light waves is very difficult to notice under ordinary circumstances because the angle of bending is so small. The bending angle is approximately equal to the ratio of the wavelength of the light to the size of the hole, an angle that is usually less than 10.sup.-3 (one one-thousandth) of a degree. A standard criterion called the "Rayleigh range" identifies the distance over which a collimated beam remains well defined after passing through a hole with a given cross sectional area. The Rayleigh range is the ratio of the area of the hole to the wavelength of the light. The Rayleigh range (here denoted Z) is mathematically characterized by the formula Z=A/.lambda., where A denotes the hole's area and .lambda. denotes the light's wavelength. For visible light .lambda. is very small, in the range 15-30 millionths of an inch. A circular hole with a radius equal to one inch has a Rayleigh range of about Z=2 miles. For this reason the diffraction illustrated in FIG. 1 will ordinarily be simply undetectable.
However, if an attempt is made to define the beam extremely well (to be able to illuminate a very small spot quite precisely) then the situation is very different. A spot radius of 50 microns (about two-thousandths of an inch) or smaller is conceivable in applications of modern optical technology. The Rayleigh range for a beam formed by passage through a 50 micron sized hole is only one inch or less. This is much greater than the depth of field of a normal sized lens focal spot, but is still very small on a practical working scale.
These estimates indicate that current techniques for creating narrow collimated beams are simply unable to generate beams that have any significant range at all, particularly with respect to commercial operations such as drilling, embossing, scribing, testing, and other manufacturing or laboratory activities that might advantageously use very narrow beams.
The present invention appears to have applicability and utilization in the semiconductor industry in areas of high precision instruments for optical surface treatments such as etching and marking operations. In these applications, the ability of ordinary light beams to achieve near-wavelength resolution without concern about depth of field or beam divergence could be applied to high-volume integrated circuit manufacturing operations. Tolerances unknown in wafer processing without electron beam or x-ray techniques could be met with ordinary light, perhaps to great advantage in reducing capital costs, magnetic field sensitivity, and worker protection requirements, while increasing instrument reconfiguration flexibility and reducing deadtime between job-runs.
Additionally, in the area of high precision process diagnostics, a major change is evolving in process-flow diagnostic instruments. A new generation of instruments uses laser probes to tag (by excitation of fluorescence, for example) molecules participating in a flowing or mixing process at very precisely located highly sensitive regions of the process. The input probe and the signal received back from the light-sensitized molecule are optical and do not disturb the flow or mix in any way. This is in contrast to all of the previous methods that use mechanical sensors inserted into the process, or macroscopic markers or floats injected to accompany the process. These prior art approaches have the disadvantage that their presence necessarily disturbs the environment being measured. The purpose of localized observations is to provide early warnings of turbulent flow, to monitor the degree of completion of a reaction, etc. The present invention has the advantage of allowing highly precise positioning of its beam center and immunity against beam divergence over relatively great depth of field, compared with all other prior art laser devices.